My favorite exercise for golf is the seated medicine ball twist. Get yourself a medicine ball and sit on the floor with your knees up. Hold the medicine ball out in front of you and slowly rotate your shoulders to the right making sure the medicine ball is still in front of you. Then rotate your shoulders as far as you can to the left. Again making sure the medicine ball is directly in front of you and not off to the side. This exercise will build core strength as well as flexibility to help you with your rotation and backswing.